Astronauts explore a new world in Interstellar. Photo: Paramount/Warner Bros.
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Near-light speed travel increasingly impossible, according to maths

Travelling at close to the speed of light may be necessary for humans to colonise the galaxy, but the maths show it'd be like flying through a cloud of bombs - but also that we should notice the explosions here on Earth, if any other civilisation has managed the feat.

The sci-fi blockbuster Interstellar depicts a dilapidated Earth too tired to support life anymore. To survive, mankind sends a team of researchers on a spaceship through a wormhole to find a new home in another galaxy, thus preserving possibly one of the rarest things in the universe – intelligence. This sounds fanciful, but it could happen in real life  engineers say that future technologies may make a spacecraft capable of interstellar travel possible.

No matter how badly we treat the planet, the Sun will be responsible for its ultimate destruction. After roughly one billion years the Sun will have grown into a red giant, and the gradual increase in temperature will have wiped clean the Earth’s surface. There are three options (assuming our species isn’t extinct before then): 1) sit and twiddle our thumbs until we are fried to death; 2) move the Earth far away from the Sun (which would bring a different set of problems); or 3), find a new home.

It seems the third option is the only ideal choice we have. We'll have to start building interstellar spacecraft eventually, and the lenient deadline of one billion years should give us enough time to do so. Fairly straightforward calculations tell us spacecraft capable of travelling at a significant fraction of the speed of light is possible - in so-called "relativistic spacecraft"  with enough time for technological advancement, and, of course, money.

There are of course plenty of challenges, though, and Ulvi Yurtsever and Steven Wilkinson from defence contractor Raytheon outline one which until now has been overlooked. In a paper published in arXiv, they say that any object travelling at relativistic speeds will interact with photons in the cosmic microwave background (CMB), creating a drag that results in slower travel.   

The CMB is the afterglow of the Big Bang, present in every direction that we point our telescopes as a faint light occupying the microwave part of the electromagnetic spectrum. Each cubic centimetre of the cosmos has over 400 microwave CMB photons, so any spacecraft moving through these would have a difficult time avoiding them. It would be like trying to dodge a swarm of flies on the driveway; you will get some goo on your windshield. And in this case plenty – trillions per second for a spacecraft moving at significant fractions of the speed of light.

Particle physics dictates that if the energy involved in a collision between an atom's nucleus and a microwave photon is high enough, electron-positron pairs can be created. An electron-positron pair is when a high energy photon (a packet of energy) interacts with a heavy nucleus to form a positively charged electron – a positron.

Yurtsever and Wilkinson describe how CMB photons will appear, from the perspective of the spacecraft travelling close to the speed of light (known as its "rest frame"), as highly energetic gamma rays that have a range of effects. If those photons interact with the material of the spacecraft hull, the effects will range from ionisation to "Compton scattering"  the scattering of high energy photons from a charged particle at rest, which in this case means further gamma rays, creating electron-positron pairs. Each time one of these pairs is formed, it creates a massive amount of energy - as much as 1.6 x 10-13 joules per pair. This doesn’t seem like a lot, but a spacecraft can collide with trillions of CMB photons per second. Assuming an effective cross-sectional area of, say 100 square metres, the effect is about 2 million joules per second across the face of the ship. That's roughly equivalent to the energy released when half a kilo of TNT explodes, every second.

Things also get more complicated when taking time dilation into account. Seconds last longer when something travels nearly as fast as the speed of light, relative to something travelling at a slower speed, so our theoretical spaceship will take longer to disspiate the energy that builds up on its front  increasing the effective energy hitting it per second to somewhere in the order of 1014 joules, or a little bit more energy than that released by the atomic bomb which fell on Hiroshima.

So, travelling at almost the speed of light will obviously have a huge drag effect. Yurtsever and Wilkinson write that a way to overcome the issue would be to keep the spacecraft’s velocity below the threshold for electron-positron pair creation, thus reducing drag and energy dissipation. That threshold is crossed as the spacecraft reaches 99.9999999999999967 per cent of the speed of light, so it's still a relatively high velocity.

There's an interesting side effect to all this, though  any relativistic spacecraft like this will bounce into so much of the CMB, it'll scatter it in a way that produces a unique light signature. "As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology," write Yurtsever and Wilkinson. In other words, if we know what to look for, we should be able to spot the interstellar contrails of near-light speed spaceships.

They actually calculate the properties of this signature  it should take the form of radiation in the terahertz to infrared regions of the electromagnetic spectrum, and it should also be moving relative to the rest of the CMB. If relativistic spacecraft are darting through the cosmos, this kind of signature should be visible using current astrophysical observatories.

However, Yurtsever and Wilkinson also look at what would happen to such a ship if it hit anything bigger than a photon  like, say, a grain of dust. Collision with an object as tiny as 10-14 grams would have an impact energy close to 10,000 megajoules, which makes it clear that any relativistic spacecraft would need a clear runway before it can take-off to a new land for the sake of humanity. Or perhaps this is just yet more evidence that, like the crew in Interstellar, jumping between wormholes is a better bet.

Tosin Thompson writes about science and was the New Statesman's 2015 Wellcome Trust Scholar. 

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“Like a giant metal baby”: whether you like it or not, robots are already part of our world

For centuries, we have built replacements for ourselves. But are we ready to understand the implications?

There were no fireworks to dazzle the crowd lining the streets of Alexandria to celebrate Cleopatra’s triumphant return to the city in 47BC. Rather, there was a four-and-a-half-metre-tall robotic effigy of the queen, which squirted milk from mechanical bosoms on to the heads of onlookers. Cleopatra, so the figure was meant to symbolise, was a mother to her people.

It turns out that robots go back a long way. At the “Robots” exhibition now on at the Science Museum in London, a clockwork monk from 1560 walks across a table while raising a rosary and crucifix, its lips murmuring in devotion. It is just one of more than 100 exhibits, drawn from humankind’s half-millennium-long obsession with creating mechanical tools to serve us.

“We defined a robot as a machine which looks lifelike, or behaves in lifelike ways,” Ben Russell, the lead curator of the exhibition, told me. This definition extends beyond the mechanisms of the body to include those of the mind. This accounts for the inclusion of robots such as “Cog”, a mash-up of screws, motors and scrap metal that is, the accompanying blurb assures visitors, able to learn about the world by poking at colourful toys, “like a giant metal baby”.

The exhibits show that there has long existed in our species a deep desire to rebuild ourselves from scratch. That impulse to understand and replicate the systems of the body can be seen in some of the earliest surviving examples of robotics. In the 16th century, the Catholic Church commissioned some of the first anthropomorphic mechanical machines, suggesting that the human body had clockwork-like properties. Models of Jesus bled and automatons of Satan roared.

Robots have never been mere anatomical models, however. In the modern era, they are typically employed to work on the so-called 4D tasks: those that are dull, dumb, dirty, or dangerous. A few, such as Elektro, a robot built in Ohio in the late 1930s, which could smoke a cigarette and blow up balloons, were showmen. Elektro toured the US in 1950 and had a cameo in an adult movie, playing a mechanical fortune-teller picking lottery numbers and racehorses.

Nevertheless, the idea of work is fundamental to the term “robot”. Karel Čapek’s 1920s science-fiction play RUR, credited with introducing the word to the English language, depicts a cyborg labour force that rebels against its human masters. The Czech word robota means “forced labour”. It is derived from rab, which means “slave”.

This exhibition has proved timely. A few weeks before it opened in February, a European Parliament commission demanded that a set of regulations be drawn up to govern the use and creation of robots. In early January, Reid Hoffman and Pierre Omidyar, the founders of LinkedIn and eBay respectively, contributed $10m each to a fund intended to prevent the development of artificial intelligence applications that could harm society. Human activity is increasingly facilitated, monitored and analysed by AI and robotics.

Developments in AI and cybernetics are converging on the creation of robots that are free from direct human oversight and whose impact on human well-being has been, until now, the stuff of science fiction. Engineers have outpaced philosophers and lawmakers, who are still grappling with the implications as autonomous cars roll on to our roads.

“Is the world truly ready for a vehicle that can drive itself?” asked a recent television advert for a semi-autonomous Mercedes car (the film was pulled soon afterwards). For Mercedes, our answer to the question didn’t matter much. “Ready or not, the future is here,” the ad concluded.

There have been calls to halt or reverse advances in robot and AI development. Stephen Hawking has warned that advanced AI “could spell the end of the human race”. The entrepreneur Elon Musk agreed, stating that AI presents the greatest existential threat to mankind. The German philosopher Thomas Metzinger has argued that the prospect of increasing suffering in the world through this new technology is so morally awful that we should cease to build artificially intelligent robots immediately.

Others counter that it is impossible to talk sensibly about robots and AI. After all, we have never properly settled on the definitions. Is an inkjet printer a robot? Does Apple’s Siri have AI? Today’s tech miracle is tomorrow’s routine tool. It can be difficult to know whether to take up a hermit-like existence in a wifi-less cave, or to hire a Japanese robo-nurse to swaddle our ageing parents.

As well as the fear of what these machines might do to us if their circuits gain sentience, there is the pressing worry of, as Russell puts it, “what we’re going to do with all these people”. Autonomous vehicles, say, could wipe out the driving jobs that have historically been the preserve of workers displaced from elsewhere.

“How do we plan ahead and put in place the necessary political, economic and social infrastructure so that robots’ potentially negative effects on society are mitigated?” Russell asks. “It all needs to be thrashed out before it becomes too pressing.”

Such questions loom but, in looking to the past, this exhibition shows how robots have acted as society’s mirrors, reflecting how our hopes, dreams and fears have changed over the centuries. Beyond that, we can perceive our ever-present desires to ease labour’s burden, to understand what makes us human and, perhaps, to achieve a form of divinity by becoming our own creators. 

This article first appeared in the 23 March 2017 issue of the New Statesman, Trump's permanent revolution